Zero-valent rhodium catalyst hydrogenation

ABSTRACT

A process for the preparation of novel zero-valent rhodium catalysts. The catalysts are prepared by the reaction of a hydrocarbyl-lithium compound with a hydrocarbon-soluble complex of a rhodium halide and a ligand. The catalyst may, if desired, be deposited on a support such as alumina or silica. It is effective to catalyze the hydrogenation of organic compounds such as benzene, styrene and the like.

This is a division of application Ser. No. 10,459 filed Feb. 8, 1979,which is a continuation-in-part of Ser. No. 827,278 filed Aug. 24, 1977and now U.S. Pat. No. 4,152,303.

The invention of this application relates to a novel catalyst and itsmethod of preparation. More particularly, it relates, as indicated, to azero-valent rhodium catalyst; the catalyst is effective to promote thehydrogenation of aromatic, olefinic and acetylenic compounds, as well asother reactions normally susceptible to catalysis.

Ten Hoedt et al., J. Organomet. Chem. 133 (1977) 113-121, show thepreparation of certain mixed-organocopper cluster compounds by theligand substitution reaction of Ar₄ Cu₆ Br₂ with two equivalents ofLiC-CR.

Popov et al., C.A. 84: 73771w, suggest that the effectiveness of arhodium-alumina catalyst in the hydrogenation of benzene is directlyproportional to the proportion of rhodium in the catalyst. Thetemperature of the hydrogenation ranged from 100° C. to 160° C.

Alchudzhan et al., C.A. 71: 12280v, studied the temperature dependenceof the rate of benzene hydrogenation on the rhodium-silica catalyst. Theactivity was studied at 200° C., 160° C., 140° C., 115° C., 90° C. and70° C. The activity-temperature curve showed a maximum at 110° C. Also,the activity of Group VIII metals was shown to decrease in the seriesrhodium-ruthenium-platinum-palladium.

Alchudzhan et al., C.A. 72: 66452s, show the catalysis of benzenehydrogenation by a silver rhodium mixture and also by a rhodium goldmixture.

Bryce-Smith et al., Ger. Offen. No. 2,117,439, show the preparation ofimproved transition metal catalysts by treating a salt of the metal withthe adduct of an aromatic compound and an alkali metal or alkaline earthmetal (other than magnesium).

The invention of the present application is a process for thepreparation of a zero-valent rhodium catalyst comprising reacting ahydrocarbyl lithium compound with a hydrocarbon-soluble rhodium halidecomplex of the formula RhX_(a) L_(b) wherein X is chlorine or bromine, Lis an olefinic hydrocarbon ligand, a is 1-3 and b is 1-4, in ahydrocarbon solvent. The invention also includes the zero-valent rhodiumcatalyst thus prepared and its use in the catalysis of hydrogenationreactions. The process preferably is carried out in a dry, oxygen-freeatmosphere. The atmosphere may be, e.g., nitrogen, ethylene or argon.

The hydrocarbyl lithium compound may be represented by RLi, where R isaromatic or aliphatic. Typical aromatic radicals include o-tolyl,m-tolyl, p-tolyl, phenyl, 2,4-xylyl, 1-naphthyl, 2-naphthyl, etc.Typical aliphatic radicals include methyl, ethyl, n-propyl, isopropyl,n-butyl, sec-butyl, tert-butyl, n-pentyl, n-hexyl, 3-hexyl,2-methyl-4-pentyl, etc. Aromatic hydrocarbyl radicals preferably have6-12 carbon atoms; aliphatic hydrocarbyl radicals preferably have 1-6carbon atoms.

The hydrocarbyl lithium compounds may be prepared by the reaction oflithium metal with the appropriate aliphatic or aromatic chlorine orbromine substituted hydrocarbon. Thus, the reaction of lithium withbromobenzene yields phenyl lithium.

The rhodium halide complex has the composition indicated by the formulaRhX_(a) L_(b). The halide may be chlorine or bromine. Chlorine ispreferred. The olefinic hydrocarbon ligand (L) is construed broadly;specific illustrative embodiments include ethylene, cyclooctadiene-1,3,norbornadiene, butadiene, isoprene, 1-hexene, and the like. Olefinichydrocarbons having up to 12 carbon atoms are contemplated. Examples ofrhodium halide complexes include RhCl(CH₂ ═CH₂)₂, RhBr.cyclooctadiene,RhCl.norbornadiene, etc. A method for preparing the rhodium chlorideethylene complex is shown by R. Cramer, J. Am. Chem. Soc., 86: (1964)217.

The process is carried out very simply, merely by mixing the reactantsat room temperature, i.e., from about 20° C. to about 30° C. Thetemperature is not a critical feature of the process and lower or highertemperatures may be used depending, for the most part, on the particularreactants which are used. A reaction occurs at once. The zero-valentrhodium product may be used as such in a catalytic hydrogenation, or itmay be deposited on a support and isolated by decanting the hydrocarbonsolvent away from the solid product. The support may be any of thosecommonly used in catalytic chemistry, viz., alumina, silica, clay andthe like.

The process is carried out in a solvent. The reactants may not becompletely soluble in the solvent, and the zero-valent metal product isnot soluble so that agitation of the process mixture is highlydesirable. Suitable solvents include benzene, toluene, xylene,ethylbenzene, pentane, cyclohexane and, in fact, any hydrocarbon solventwhich is normally liquid, i.e., liquid at about room temperature.

Relative proportions of the rhodium halide complex and thehydrocarbyl-lithium are indicated by the stoichiometry of the reaction.Care should be taken not to use an excess of the hydrocarbyl-lithiumcompound in which case the resulting product shows no catalyticactivity. At the same time, however, addition of a small amount of waterto such a product is effective to convert it to an efficient catalyst.

The hydrogentation reactions which are catalyzed by the zero-valentmetal products herein may in most instances be carried out at roomtemperature and at ordinary pressures. Aromatic compounds, i.e., thearomatic ring, can be hydrogenated merely by introducing hydrogen into areaction vessel containing the aromatic compound and the catalyst.Benzene and naphthalene, for example, can be hydrogenated in thisfashion, benzene yielding cyclohexane and naphthalene yielding a mixtureof cis- and trans-decalin. Olefinic compounds can also be hydrogenatedunder similar conditions. Styrene, for example, can be converted toethylbenzene and then to ethylcyclohexane. Stilbene can be converted to1,2-diphenylethane and then to 1,2-dicyclohexylethane. Phenylacetylenecan be hydrogenated likewise to ethylbenzene, and then toethylcyclohexane.

The activity of the hydrogenation catalyst varies with its concentrationin the hydrogenation mixture. In benzene, for example, the activity,measured in ml. of hydrogen absorbed per minute per mmol of zero-valentrhodium, increases as the catalyst concentration decreases from 0.067mmol of Rh/ml. (of benzene) to 0.008 mmol of Rh/ml., then decreases uponfurther decrease of catalyst cencentration.

In Example 5 of the above-identified Application Ser. No. 827,278, filedAug. 24, 1977, now U.S. Pat. No. 4,152,303 there is described thepreparation of a zero-valent gold-rhodium product, as follows: To asuspension of 0.5 mmol of p-tolylgoldlithium etherate (p-tol)₄ Au₂ Li₂(C₂ H₅ OC₂ H₅)₂ in 30 ml. of benzene under nitrogen, there is added withstirring 1.0 mmol of rhodium chloride ethylene complex (RhCl.(CH₂═CH₂)₂). Immediately, the color of the suspension darkens, indicatingthe formation of (p-tol)₄ Au₂ Rh₂, followed immediately by itsdecomposition to the desired Au°/Rh°.

EXAMPLE 1

The preparation of Rh° may be accomplished in a similar manner, reactingp-tolLi with a rhodium halide complex such as the ethylene complex. Anunstable intermediate similar to the above polymetal structure is formedand then the desired Rh° metal.

EXAMPLE 2

A rhodium chloride-cyclooctadiene complex is prepared by the method ofJ. Chatt et al., J. Chem. Soc., (1957) 4735. To a solution of 10 g. ofrhodium trichloride hydrate.(RhCl₃.3H₂ O) in 300 ml. of ethanol there isadded 21 ml. of cyclooctadiene and the resulting mixture heated atreflux temperature for three hours. The cooled mixture is filteredyielding the solid rhodium chloride-cyclooctadiene complex, (RhCl.COD)₂,M.P. 249°-250° C.

EXAMPLE 3

A solution of 30.67 mg. (0.125 mmol) of rhodium chloride.cyclooctadienecomplex in 15 ml. of benzene, under nitrogen, is treated at roomtemperature with 0.008 mg. (0.125 mmol) of n-butyl lithium. Immediately,the mixture is flushed with hydrogen. A slightly exothermic reactionensues and the right yellow solution is converted to a dark brownslurry.

The catalytic activity of the above resulting zero-valent rhodiumproduct slurry is determined by attaching to the reaction flask, througha stopcock, a gas burnt filled with hydrogen and equipped with aleveling bulb. The nitrogen is evacuated and the flask filled withhydrogen to atmospheric pressure (744 mm. ) and the contents of theflask shaken vigorously. The hydrogenation of the benzene is monitoredby observing the decrease of gas volume in the buret. The rate ofhydrogenation is found to be 1120 ml./min./mmol of zero-valent rhodium.

Naphthalene, stilbene, styrene, phenylacetylene and other aromatic,olefinic and acetylenic compounds may be hydrogenated similarly.

All parts and percentages herein are by weight unless otherwiseexpressly stated.

We claim:
 1. A process for the hydrogenation of aromatic, olefinic andacetylenic compounds comprising treating such compounds with hydrogen inthe presence of a zero-valent rhodium catalyst prepared by a processcomprising reacting a hydrocarbyllithium compound with a rhodium halidecomplex of the formula Rh X_(a) L_(b) where X is a chlorine or bromine,L is an olefinic hydrocarbon ligand, a is 1-3 and b is 1-4 in ahydrobcarbon solvent.
 2. The process of claim 1, wherein thehydrogenation is carried out at about room temperature.